This invention relates generally to screw presses for expressing fluids from fibrous materials, and more particularly to a bi-metallic feed screw or worm for use in conjunction with screw presses.
The worm flights which radially extend from the worm of feed screws of high pressure expressing presses incur substantial wear and abusive interaction with both the fibrous material and debris contained therein as it interacts with the walls of the screw press. It is a well-known technique to provide wear resistant or hard-facing coatings upon the surfaces of the worm flight which are subjected to highest wear. Techniques utilized for this purpose are deposit welding, flame spray deposition, plasma deposition and the like. Thereafter, the surfaces are smoothed manually back to the desired dimension of the .flight. These conventional deposit welding techniques are labor intensive, require expensive components, and provide poor bonding between the ductile base material and the harder deposit weld material.
An effort to resolve this wear problem is disclosed in U.S. Pat. No. 3,592,128 to French which teaches the addition of hardened insert members removably connected to the forwardly facing surface of each worm flight. Another effort to resolve peripheral wear is disclosed in U.S. Pat. No. 4,223,601 to Knuth et al. which discloses a worm flight formed with an outwardly opening groove which is filled by deposit welding with a harder metal than that of the flight. Yet another effort to resolve the peripheral worm flight wear problems is disclosed in my earlier U.S. Pat. No. 4,440,076 wherein a plurality of hardened inserts are embedded in a row outwardly extending at the periphery of the worm flight.
Several attempts have also been made to produce a homogeneous feed screw by utilizing casting techniques. However, if a highly wear resistant, brittle material is chosen, cracking at the key way or other overall highly stressed areas occurs. Alternatively, where a more ductile material is used, premature wear of the flight is experienced.
The present invention provides an optimal utilization of both ductile and brittle casting material by providing a bi-metallic structure rigidly interconnected to form a worm flight of brittle material and a worm body or collar of more ductile material. Various embodiments and methods of manufacture are also provided utilizing casting techniques.